The invention relates to a method of indoor radio transmission for the parallel radio transmission of digital substreams formed by space-time encoding as a function of a selected data modulation format relative to a number n of data signals, in a multi-path transmission channel of limited frequency between a number of n transmission paths and a number m (m≦n) reception paths positioned at various sites within the indoor space with a measurement of the elements of the complex valued channel matrix for estimating the space-time-behavior of the transmission channel and space-time signal processing in the receiving paths while determining a weighting matrix derived from the complex valued for the simultaneous recovery of the data subsignals which by space-time encoding are again formed into digital data substreams, and to a mobile radio transmission system.
Wireless transmission channels are not without flaws. Echoes of signals at the receiver result in superimposition and interferences which limit the useful band or channel width of a carrier modulated with data to a value below the frequency range licensed to an individual offeror. Overcoming this channel limitation constitutes a first step towards higher transmission capacities in wireless networks. A method currently employed to this end is multi-carrier modulation in which the transmission capacity is in principle only limited by the permissible frequency range. Yet in future, wireless networks are to be available for many users in commerce and industry at data rates of 100 Mbit/s, similar to those known from land-line networks. However, the licensed frequency range cannot accommodate this. Wireless transmission systems which spatially and chronologically process transmission signals and which are thus of a high spectral efficiency, appear in this context to offer the best possibilities for further increasing the capacity of wireless networks.
Conventional receivers make use of omnidirectional antennae which in addition to the direct transmission signals (“line of sight” LOS) collect a number of echoes. Depending upon the receiving position, these superimposed signals are of different amplitudes and phases, so that the total signal is subject to a strong spatial variation known as “fading”. Fading constitutes the problem typical of wireless transmission which conventionally is sought to be overcome by a multiple so-called “diversity reception”. Lately, “frequency expansion techniques” are being applied which make use of the fact that fading is a frequency selective narrow-band phenomenon. Broadband signals can at least partially compensate the spatial signal changes. In modern transmission systems the carrier is modulated by digital signals. The direct signal and its echo reach the receiver with different delays. Significant cross-talk of successive data bits is only detected if the bit duration approaches the range of the transmission expansion τdelay which is defined as the second static moment in the channel pulse response and which is inversely proportional to the channel band width B, calculated as
  B  =      1          2      ⁢              πτ        delay            
The bandwidth useful for the modulation is limited to about the channel bandwidth B. In normal indoor spaces, the value may be at a few megahertz, particularly when the direct signal is absent. Superfast data transmission on a single carrier frequency may therefor be realized by a modulation scheme of high spectral efficiency. This, however, leads to a reduced sensitivity of the reception.
By exceeding the limits of the channel, the transmission capacity can be increased by the use of several carrier frequencies. Orthogonal frequency division multiplexing (OFDM) is a method often used to modulate several carrier frequencies. In this, individual transmitted symbols may have extreme peak values which can only be transmitted by dear broadband amplifiers of high linearity. But the spectral limits can also not always be reached by OFDM. For instance, in the ETSI HIPERLAN/2-Standard, OFDM subcarriers are used within a bandwidth of 20 MHz. The entire spectrum from 200 or 255 MHz in accordance with the lower and upper 5-MHz-ISM-frequency band is not available for individual users, since in normal circumstances only of nineteen nominal carrier frequencies is used for transmission. Depending upon the channel quality, the data rate is set, by changing the spectral efficiency of the OFDM-subcarrier modulation, to values from 6 Mbit/s to 54 Mbit/s [1]. If, however, in a commercial environment more than nineteen users are connected to a base station, the available time window and, hence, the data rate per user, is but a fraction of these values. A possible relief is to increase the number of base stations by reducing the range covered by them. Overall, a comprehensive optimization of transmission conditions for indoor applications is predicated on an increase of the infrastructure.
A new degree of freedom for the capacity of the system is achieved by spatial diversification. For instance, in a tracked directional link the user's position is tracked by transmission of a directional beam. Multiple connections of base stations with mobile users at different locations may be served by the simultaneous use of equal frequencies. Directional beams reduce the number of undesired links, echoes and other interference signals. The advantages of directional antennae thus improve the quality of the links. In sight link arrangements, base band processing is also simpler than it is in an OFDM. Owing to the reduced number of signal echoes, the sight links makes available the entire band width of the permissible spectrum for modulating a single carrier frequency. However, this requires the power beamed in by the sight link to be significantly higher than from all other signals. The required beam width typically is 10° in order to raise the “Rice factor” K (Power ratio of the signal of the sight link relative to the sum of the signals of all echoes) above the steep threshold at 15 dB, at which the transmission expansion is abruptly reduced [2].
Directional beams may be provided, for instance, by horn aerials aimed at the base station. However, mobile users may not easily accept the alignment procedures (tracking) required for the network connection. A system for the automatic positional determination and alignment, especially at the base station, appears to be rather more promising. Thus, there have been attempts involving arrays of aerials in which the complex valued amplitude of each element is measured. From this, special signal processing algorithms can calculate the direction of the incoming planar wave of the sight link. Each areal signal is then multiplied by a complex weighting, and all signals are summed. This linear combination of the areal signals is physically equivalent to directing a beam directed to a target positioned in the desired direction) beam formation) [3].
This proposal for solving the problem appears not to be practicable in connection with wireless networks in indoor spaces. A large number of antenna elements (10×10) is required in order two-dimensionally to achieve a beam width of 10°, and the requirements put on the signal processor for defining an tracking a position are very high. Trials with digital signal processors for accommodating high data rates have failed at only a fraction of the required antenna elements. The processor design currently possible allows data rates of 1.5 Mbit/s with eight antenna elements [4]. In view of these results it does not seem possible in the foreseeable future with 10×10 antenna elements to achieve a data rate of 100 Mbit/s.
For high data rates, hardware implementations will thus have to be taken into consideration for the processing of data. An approach for simplifying signal processing which makes use of several preformed signal beams is realized by a hardware-configured matrix in accordance with Butler placed in front of the array of aerials. This allows realization of an array of antennae of a switchable signal beam. The receiving direction will then be found by scanning all outputs of the Butler matrix for the best signal which is then switched through to the receiver. The complexity of the Butler matrix quickly increases, however, as a function of the number of outputs. Each antenna signal is individually delayed before it reaches an output and is then summed with all other signals. For that reason, a Butler matrix with 10×10 outputs would require 10,000 delay branches. Realization of such a network would appear not to be reasonable. Moreover, a 10×10 antenna array for 5 GHz which at a space of 3 cm (λ/2) between antennae covers an area of 30×30 cm2, is too large for mobile stations so that the described technology is either reserved for stationary base stations or must be limited to mm wavelengths.
The state of the art upon which the invention is based proceeds from the transmission method and transmission system according to Foschine et al. as described in EP-A2 0,951,091 and the [5] and [6] papers. The wireless transmission system for indoor applications known therefrom (for better distinction hereinafter referred to as “BLAST” method) is based on time-space signal processing for a transmission system having a plurality of antennae on its transmission and receiving side (multi-element antennae, multiple-input/multiple-output—MIMO— system). However, it is not the bandwidth of the transmission channel which is enlarged in the BLAST method, but the spectral efficiency as a measure of the data rate transmissible within a band width of 1 Hz. In it, spectral efficiency and transmission capacity increase linearly with the number of parallel transmission paths. The principle of the MIMO data transmission may be gleaned from FIG. 1 for n transmission antennae and m receiving antennae with a total of n×m parallel transmission paths. On the transmission side an incoming data stream is divided by a space-time encoder into a number corresponding to the number of transmission antennae. These are wirelessly transmitted as data signals chronologically and spatially parallel on the same carrier frequency. At the receiving side the superposed data signals are detected at different positions of the indoor space by receiving antennae the number of which is greater than or equal to the present transmission antennae (m≦n). In this connection, the indoor space, being a short distance transmission range, displays normal transmission behavior free of unusual superpositions, strong attenuations and interferences by moving objects. Nevertheless many echo signals contribute to the transmission signal. For that reason it may be assumed that at sufficient distance between the antennae the received signals are statistically independently and randomly distributed (distribution in a sight link according to Rice or Raleigh where no sight link is given).
In the following explanations which contribute to the basic understanding of the BLAST method upon which the invention is based, the transmitted signals are designated by the transmission vector S with components S1. After wireless transmission the components E1 of the receiving vector E are calculated as
      E    i    =                    ∑                  j          =          1                n            ⁢                        H          ij                ⁢                  S          j                      +          N      i      where Hij are elements of the complex valued channel matrix [H] and Ni is the noise contribution in the ith receiving path. The channel matrix estimates the actual transmission behavior of the indoor space as a multi-path transmission channel. In the BLAST method training vectors are transmitted periodically to determine the channel matrix. Since in a natural environment, uncorrelated transmission signals are very probable, in most cases a number of linear equations, which are independent of each other, corresponding at least to the number of transmission antennae, may be established. In this manner, the channel matrix receives all the information necessary for reconstructing, on the receiving side, the transmission signal from the received data signal. In the known wireless transmission method, this is carried out by singular eigenvalue division (SVD) and subsequent iterative extinction of interference.
The BLAST method is based on a special physical model. The method successfully utilizes the non-frequency selective so-called “flat fading” for improving the system capacity. In indoor space represents a microwave resonator with partially reflective surfaces. The modes of this resonator differ in their spatial field distribution as well as in their center frequency. One carrier frequency usually excites about 10,000 modes (the spectral line width of a single mode is comparable to the channel band width B). As a result of the mode field distribution, every transmitter excites different linear combinations of modes which at different sites of the space where the receivers are positioned, additionally blank each other with different amplitude and phase and are thus linearly independent. The BLAST method utilizes these spatially conditioned differences in the composition of the receiving signals for reconstructing the original signals.
At a high signal to noise ratio, the BLAST method operates with eight transmission antennae and twelve receiving antennae and a sixteen QAM transmission (QAM: quadrature amplitude modulation) as the selected data modulation format. This is said to lead to spectral efficiencies of up to 40 bit/s per Hz at bit error rates in the range of 1%. However, for measuring the channel matrix and data transmission, the known method utilizes alternating time periods, so that 20% of the achievable effectiveness is lost again. In addition, space-time processing in real time is performed with a digital signal processor which limits the data throughput to less than 1 Mbit/s. Yet it would be possible, with the eight parallel transmission channels in indoor spaces to achieve a total data throughput of several 10 Mbit/s up to 100 Mbit/s and more. However, in the near future it seems unlikely that such data rates will be achieved with data processing based on a digital signal processor, such as the known BLAST method. A high data rate is, however, of particular interest for multimedia applications (videos, computer communications, etc.) or a basic requirement for their applications. Mobile communications systems have to be frugal with the frequency resources allotted to them. As has already been mentioned, however, a high data rate for multimedia applications is desired at the same time, especially in the so-called indoor area. This requires a high spectral efficiency which must be paired with an efficient transmission technology. Because of the multiple reflections of the wireless waves, the channel band width is typically restricted to about 5 MHz. Proceeding from the known BLAST method as a similar method, which already requires fewer antenna arrays than directional sight links, it is thus an object of the method according to the invention within this “natural;” band width to transmit data at very high transmission rates in a range of 100 Mbit/s. This requires achieving a spectral efficiency of 20 bit/s per Hz. A suitable transmission system for executing the method must be correspondingly efficient and be capable of signal processing at very high rates.